ceph-osd is the object storage daemon for the Ceph distributed file
system. It is responsible for storing objects on a local file system
and providing access to them over the network.

The datapath argument should be a directory on a xfs file system
where the object data resides. The journal is optional, and is only
useful performance-wise when it resides on a different disk than
datapath with low latency (ideally, an NVRAM device).

Flush the journal to permanent store. This runs in the foreground
so you know when it’s completed. This can be useful if you want to
resize the journal or need to otherwise destroy it: this guarantees
you won’t lose data.